Papers by D. W. Snow

Today we’re going to go back in time, all the way to the the dawn of computer networks. When this technical report was filed, the largest operational internetwork was still called ARPAnet and it still ran on NCP; people were still talking about hosts and communications subnetwork processors as if they were two different physical devices, and security levels as if that was the only possible way to conceptualize access control; and I was five months old.

(I apologize for the poor quality of the linked PDF—to the best of my knowledge, this is the only version to be found online. Also, if anyone knows D.W. Snow’s first name, please tell me.)

To the best of my knowledge, this is the very first published article to discuss the things that end-to-end encryption (what we would now call a secure channel protocol) does not protect from an eavesdropper. Everyone doing computer security in 1978 was thinking mostly about protecting classified government secrets, so the authors frame the problem in terms of a Trojan Horse program with access to such secrets, but forbidden by the OS from sending messages to anyone who isn’t cleared to access the same secrets: if all outgoing network traffic is encrypted end-to-end to its (legitimate) recipient, can this Trojan Horse still exfiltrate information to someone who isn’t a legitimate recipient?

They point out that, of necessity, a packet-switched network has to reveal the destination address, transmission time, and length of every packet in cleartext. They model each of these as Shannonian communication channels, and determine sending rates on the order of 100 bits per second for each—more than adequate to leak a text document. (They observe, by way of comparison, that the standard military teletype runs at 75 bps.)

Nowadays, this threat model might seem quaint, even silly—we put a lot more effort into preventing untrusted code from seeing secret information in the first place. The information leak, however, is real, still exists, and can be used for other purposes. The most terrifying example I know is Hookt on fon-iks, in which a completely passive eavesdropper can reconstruct the words spoken in an encrypted VoIP phone conversation, just from the length and timing of each packet. Different syllables compress to different length packets, and every natural language has rules about which syllables can follow which; the rules can be modeled with a Markov chain, and there you are.

The countermeasures and conclusions sections of this paper are much more embarrassing in retrospect than the dated threat model. They say, there’s nothing one can practically do about this end-to-end, but we can close the hole if we make every single intermediate relay a trusted arbiter of the (one true) security policy, at which point we don’t need end-to-end encryption… I feel quite confident in saying, even in 1978 it ought to have been obvious that that was never going to happen. What’s sad is, if people hadn’t given up on end-to-end countermeasures back then, perhaps we would actually have some by now. (It’s easy for VoIP! All you have to do is use a constant-bitrate compression algorithm. Shame none of the widely deployed VoIP programs bother.)